This invention relates to thermally reworkable epoxy resin compositions, and more particularly to thermally reworkable carbamate or carbonate epoxide resin compositions which degrade at temperatures significantly lower than traditional cycloaliphatic epoxy resins.
Modern electronics manufacturing relies upon two general techniques to attach electrical components, such as integrated circuit chips (IC""s), resistors, capacitors and the like, to circuit boards. In the older, traditional reverse-mounting method, the components include wire leads which are extended through holes in the circuit board and soldered to connections on the back side of the circuit board. In recent years, the reverse-mounting method has been largely supplanted by the technique of surface-mounting, in which the components are soldered to the same side of the board to which they are mounted. Surface-mounting offers may advantages over the reverse-mounting method, including reduced assembly time, lower cost, and the ability to interconnect very small structures at a much higher density.
Integrated circuit (IC) chips usually have a large number of connecting leads in a very small area to support their high associated I/O requirements. Accordingly, surface-mounting techniques are well suited from the attachment of IC chips to circuit boards. One surface mounting technique which has grown in popularity in recent years is the technique known as xe2x80x9cflip-chipxe2x80x9d mounting. In the flip chip method, small solder bumps are positioned at locations on the surface of the circuit board and/or the underside of the chip wherein it is desired to form interconnections. The chip is mounted by placing it in contact with the circuit board and then heating it to cause the solder to reflow. Upon cooling, the solder hardens to attach the chip to the board and to create the appropriate electrical connections.
As initially practiced, the flip-chip technique oftentimes utilized relatively high cost materials, such as high lead solder and ceramic substrate. However, the desire to reduce costs has prompted the use of less expensive materials, such as in the flip-chip on board (FCOB) method, which typically utilizes eutectic solder and organic printed wiring board (PWB). While reducing material costs, the use of FCOB method has led to problems because of coefficient of thermal expansion mismatches between the IC chip and the organic substrate of the FCOB, particularly when large IC chips having a fine pitch and low profile solder joints are utilized. Due to the large coefficient of thermal explansion mismatch between silicon IC chips (2.5 ppm/xc2x0 C.) and organic substrates, i.e., FR-4 PWB (18-24 ppm/xc2x0 C.), temperature cycle excursions experienced by the FCOB can generate tremendous thermomechanical stress at the solder joints. Over time, these stresses can result in performance degradation of the interconnections which may degrade or incapacitate device performance.
One method developed to minimize the thermomechanical stresses on the solder joints has been to introduce an underfill material into the spaces or gaps remaining between an IC chip and substrate. The undefill is typically an adhesive, such as an epoxy resin, that serves to reinforce the physical and mechanical properties of the solder joints between the IC chip and the substrate. The underfill improves the fatigue life of the packaged system, and also serves to protect the chip and interconnections from corrosion by sealing the electrical interconnections of the IC chip from moisture. The use of an underfill can result in an improvement in fatigue life of ten to over one hundred fold, as compared to an un-encapsulated packaged system.
Cycloaliphatic epoxies, typically combined with organic acid anhydrides as a hardener, have commonly been used as underfills in flip-chip packaged systems. They offer the advantage of low viscosity prior to curing, and have acceptable adhesion properties after curing. Other epoxies such as bisphenol A or F type or naphthalene type have also been used in the underfill formulations. Silica powder has sometimes been utilized as a filler in underfill formulations in order to adjust the coefficient of thermal expansion of the underfill to match that of the solder. When the coefficients of thermal expansion of the solder and the underfill match there is much less movement and fatigue between the underside of the flip chip and the solder connections, further improving device lifetime.
By way of example, the material properties represented in Table 1 typically are exhibited by typical epoxy underfill compositions.
While the use of underfills has presented a solution to the problem of the coefficient of thermal expansion mismatch between chip and circuit board, it has created new challenges for the electronics manufacturing process. The new manufacturing steps required to apply the underfill, and to bake the assembly to harden the underfill, substantially complicate and lengthen the manufacturing process. Accordingly, it would be desirable to simplify the underfill manufacturing process for flip chips.
One method of simplifying the manufacturing process has been to dispense the underfill before placing the flip chip into contact with the circuit board using a process known as xe2x80x9cno-flowxe2x80x9d underfill. In the no-flow underfill process, the underfill is applied directly to the underside of the chip and/or circuit board before alignment of the chip on the board. Thus, when using a no-flow process it is no longer necessary to use a low-viscosity underfill material that can flow into the thin space between the chip and the circuit board. This allows the use of higher viscosity underfill materials that are easier to handle and apply than the low viscosity underfills used in more traditional flow based flip-chip manufacturing. The manufacturing process is further simplified because the heating steps for soldering and curing the underfill can be combined, eliminating several manufacturing steps.
The no-flow underfill method requires that the underfill material be adapted to allow solder interconnects to form. Generally, a fluxing agent must be applied to the solder bumps and/or the circuit pads on the circuit board to aid in interconnect formation by removing oxidation from the circuit pads and solder bumps. Accordingly, fluxing agents have been included in some prior no-flow underfill compositions to facilitate solder joint formation.
An additional disadvantage to traditional flip chip methods has been that the use of an adhesive underfill can make it difficult, if not impossible, to disassemble the components when a defect is discovered after assembly of an electrical component. Because the solder assembly and underfill steps occur simultaneously during the heating process, it is difficult to test the electronic assembly until the assembly is complete. Thus, if a defect is discovered, the underfill has already hardened, making removal and disassembly impractical. This results in increased production costs due to the waste of otherwise usable components. An effective way to address this problem is to make the flip-chip devices reworkable under certain conditions.
One method of making a reworkable flip-chip device has been to incorporate a non-stick release coating on the boundary surface between a chip and a substrate. For example, U.S. Pat. No. 5,371,328 discloses a reworkable flip-chip type of circuit module using a non-stick release coating on all surfaces intermediate of the chip and the substrate. While this non-stick release coating may be suitable in some applications, it is likely that the use of such a release coating may reduce the adhesion of all the interfaces including those of the underfill to chip and underfill to substrate. These adhesions are important to the reliability of the flip chip interconnections. Accordingly, this approach is not ideal for use in flip-chip applications.
Another approach to providing a reworkable flip-chip interconnection is to use a reworkable underfill. Presently, the materials that are undergoing development for reworkable underfills can be classified into two categories: chemically reworkable underfills and thermally reworkable underfills.
Chemically reworkable underfills generally require the use of harsh acids and/or bases. For example, U.S. Pat. No. 5,560,934, issued to Afzali-Ardakani et al., discloses epoxy compositions that are soluble in an organic acid after curing. Utilizing relatively strong chemicals such as acids (or bases) during reworking, however, oftentimes leads to a messy, time-consuming rework process. Additionally, it has been found that the use of chemicals during the rework process typically makes localized repair of a packaged system difficult and, sometimes, impossible. Therefore, it is believed that use of a thermal rework process would avoid these problems and offer the possibility of a quick, clean, and localized rework process.
U.S. Pat. No. 5,659,203, issued to Call et al., discloses a reworkable flip-chip module utilizing a specially defined thermoplastic resin as an encapsulant. The thermoplastic resin, such as polysulfone, polyetherimide, etc., possesses a high glass transition temperature (Tg), e.g., 120xc2x0 C. less than Tg less than 220xc2x0 C., and must be either dissolved in a solvent or heated above its melting point during the encapsulation process. Therefore, use of these thermoplastic resins as encapsulants for FCOB applications may be undesirable, since such applications typically require an underfill which is free of solvent and in liquid form during the encapsulation process, and typically require keeping the packaged system at lower temperatures in order to maintain the integrity of the eutectic solder which is utilized with the organic PWB.
U.S. Pat. Nos. 6,197,122 and 5,948,922, issued to Ober et al., disclose thermally reworkable underfill formulations based on thermally decomposable epoxies containing secondary or tertiary oxycarbonyl (ester) moieties. However, secondary or tertiary oxycarbonyl moieties typically can easily be cleaved by weak acid or base, and are sensitive to moisture. Also the epoxies containing secondary or tertiary oxycarbonyls typically have higher moisture uptake than a standard epoxies. All these factors tend to indicate that epoxies containing secondary or tertiary oxycarbonyl moieties might not be suitable for underfill applications where high reliability is required.
Thus, it can be seen that none of the prior art methods are ideally suited for use as a no-flow underfill to bond chip and substrate to allow fast and efficient assembly and rework of FCOB devices without sacrificing the reliability of the devices. Therefore, it is desirable to provide a no-flow underfill composition that has useful fluxing properties and which will not negatively affect the overall performance of the assembly, while still allowing cost effective and efficient rework.
Accordingly, it is an object of the present invention to provide a polymeric composition that has mechanical and fluxing properties suited to use as a no-flow underfill while also offering thermal reworkability. The present invention is focused on epoxy base materials because epoxy base materials have desirable properties for use as an underfill and are the only materials that have been proven to provide flip-chip devices with acceptable reliability.
Most epoxy materials are thermosetting compositions and are difficult or impossible to remove after curing. The present invention overcomes this limitation by developing new diepoxides that contain thermally degradable groups within their structures and using these new diepoxides in the epoxy formulations to make the thermoset network degradable at a desired temperature. This makes the new epoxy formulations reworkable. Moreover, these thermally degradable groups have good properties such as high moisture resistance, high chemical resistance and low moisture uptake so that they are suitable for underfill application. This improvement is advantageous in flip-chip application of epoxy compositions where epoxy materials are used as the underfill to reinforce the solder joints. Removal of the epoxy allows replacement of defective devices, saving the cost of discarding other valuable components in a microelectronic assembly.
There are two ways of developing reworkable epoxy base materials. One is to develop new epoxies that decompose at rework temperature. The other is to develop additives to add into the existing epoxy formulations that have previously been found suitable for use as underfill encapsulants. The present invention focuses on the first category and uses thermally degradable epoxies containing integral thermally cleavable groups that decompose at rework temperatures. The second category is the subject of U.S. Pat. No. 6,172,141.
The thermally cleavable groups of the present invention have been selected to meet the following criteria:
1. The cleavable groups should be sufficiently stable to permit the epoxy network to perform its function in a specific application;
2. The cleavable groups should be inert to the curing reaction of the epoxy network;
3. The cleavable groups should not adversely affect the overall properties of the epoxy network;
4. The cleavable groups should decompose quickly at elevated temperature so that they break down the structure of the epoxy network, leading to its easy rework.
5. The link should be stable in the environment to which the cured epoxy will be exposed.
6. The synthesis of the epoxides containing the cleavable link should be simple, with high yield, and cost effective.
The present invention discloses carbonate and carbamate epoxides which have been found to meet the above criteria. After introduction into the epoxy structure, the carbamate and carbonate groups do not significantly interfere with epoxy curing, nor do they adversely affect epoxy properties including Tg, modulus, CTE, adhesion. However, the existence of these groups inside the epoxy structure reduces the epoxy decomposition temperature from 350xc2x0 C. to as low as 200xc2x0 C. Optimal rework temperatures for flip-chip devices are generally between 200xc2x0 C. and 250xc2x0 C. because the eutectic solder reflow temperature is within this temperature region. Therefore, these two groups may be suitable for use in applications needing an epoxy which is reworkable around solder reflow temperature.
More particularly, the present invention is directed to a thermally reworkable no-flow epoxy composition for encapsulating and protecting an electronic device or assembly. The thermally reworkable epoxy composition includes the cured reaction product of: a cycloaliphatic epoxide containing either a carbonate or a carbamate group; an organic hardener; a curing accelerator; and a fluxing agent. The present invention is also directed to a method of protecting, encapsulating, reinforcing, assembling, or fabricating a device or a chemical product with a cured epoxy composition which is thermally reworkable, wherein the epoxy composition includes the reaction product of: a thermally degradable cycloaliphatic epoxide; an organic hardener; a curing accelerator and a fluxing agent.